Inversion brake valve and system therefor

ABSTRACT

A pneumatic brake system employing dual-diaphragm, spring-actuated, air-released brake actuators is provided with an inversion valve which, in the event of a brake system failure, prevents the brakes from being automatically applied while permitting controlled brake application by releasing compressed air from each actuator&#39;s spring chamber in an inverse ratio to that normally supplied from the system by the operator applied brake valve thus permitting a controlled spring brake application. The valve employs first and second pressure responsive pistons with associated valve seats to maintain the actuator springs compressed with air at supply pressure when the vehicle is normally operated. The air at supply pressure in each actuator&#39;s spring chamber is automatically ported to a lower pressure through the valve when a system failure occurs and prior to brake application to insure fast response time of the spring actuated brake.

This application is a division of application Ser. No. 611,648, filedSept. 9, 1975, now U.S. Pat. No. 4,017,125.

This invention relates generally to a brake system and, moreparticularly, to an improved operation of such brake system when apressure responsive valve of the type disclosed herein is included insuch system.

The invention is particularly applicable to a valve for use in dualcircuit pneumatic brake systems applicable to trucks. truck-tractors,buses and the like which employ dual-diaphragm, spring-actuated,air-released brake actuators and will be described with particularreference thereto. However, it will be appreciated by those skilled inthe art that the invention may have broader applications and may beemployed in vacuum or other fluid actuated brake systems.

Safety regulations have resulted in the commonplace use of brakeactuators of the dual-diaphragm, spring-applied, air-released type inpneumatic vehicle braking systems. Such actuators comprise tandem frontand rear brake chambers. The rear chamber houses a compression springwhich is maintained precompressed when the vehicle is in its normaloperating mode by air at primary, or system or supply or emergencypressures (hereinafter termed supply pressure). A forward air chamber issupplied with supply air modulated to secondary or control or service orsignal pressure (hereinafter termed service pressure) to actuate thevehicle's brakes in a conventional manner. In the event of a failure inthe air system or when the vehicle is to be parked, the rearward chamberis vented of its air to release the spring which then sets the vehicle'sbrakes. Obviously, if an air failure occurred while the vehicle wasdriven and the rear chamber was automatically vented, the sudden andfull application of the vehicle's brakes would present serious controlproblems to the operator of the vehicle.

To prevent such problems, a valve, generally known as an inversionvalve, has been employed in such systems. The function of this valve isto maintain the spring in the brake actuator compressed even though asystem failure be sensed and to vent the air from the rear chamber ofthe brake actuator in an inverse ratio to the air at service pressuresupplied to the service brake chamber by the operator through theconventional treadle valve. In this manner, the brake actuator springswould be released to exert a braking pressure which would be in "sync"with the degree of treadle valve actuation.

Heretofore, prior art inversion valves basically comprised spring loadedpiston or diaphgragm arrangements as illustrated in U.S. Pat. Nos.3,826,283 and 3,863,992. To provide a fast spring brake response time toinsure equal application of the brakes, the spring precompression of thevalve is established at a force which modulates system air pressure to avalue which is just sufficient to maintain the springs compressed in theactuators during normal operating conditions. While such valvearrangement is thus sufficient to provide quick response for actuationof the brakes, there are two significant drawbacks to such anarrangement. First, spring rates in the brake actuators vary from oneactuator to the other and the air pressure ported to the spring brakechambers may be sufficient to maintain one of the brake actuator'ssprings compressed while permitting the spring of another brake actuatorto slightly expand until equilibrium occurs. In this event, the latterspring actuator would slightly apply the brakes of the vehicle which itcontrols and thus generate heat and wear on such brakes significantlyreducing the life thereof. Second, such inversion valves by regulatingsupply air pressure to a lower value do not meet certain safety standardcriteria which require that full system air pressure be applied to theemergency line of a trailer. Accordingly, the use of prior art inversionvalves has been limited to certain vehicle applications. Specifically,they could not be used to modulate trailer emergency line pressure(emergency brakes) from the towing tractor. On the other hand, if theprior art inversion valves employed spring rates sufficiently high toinsure supply pressure in the spring brake chambers, the response timeof the spring brakes would be adversely affected with uneven brakeapplication occurring.

It is thus an object of the invention to provide in a vehicle brakesystem which employs two separate brake arrangements normally actuatingthe vehicle brakes on different axles in a simultaneous manner, afast-response inversion valve which is effective upon failure of one ofsuch systems to actuate the brakes associated with the failed system ina manner which is synchronous with the brake actuation of the other,still operable system.

The object along with other features of the subject invention isachieved in a valve employing a dual-piston, dual-valve seatarrangement. In normal vehicle operation, air at supply pressure from aline servicing those brake actuators which are to be monitored biasesthe pistons and seats in a manner which insures fluid communication froma second line, always at supply pressure, to a delivery port incommunication with the actuator's spring brake chamber. When a brakefailure occurs and prior to a brake application, supply pressure willdecrease resulting in relative piston movement to isolate the deliveryport from the second brake line. Simultaneously, a spring biasing one ofthe pistons will vent a portion of the air from the spring brake chambervia the delivery port. Venting will continue until the spring force andair pressure are in equilibrium whereat the valve will be in a lappedposition and the springs in the brake actuator just slightly compressed.When the vehicle operator actuates the brake or treadle valve, a thirdbrake line in communication with air at service pressure in the unfailedbrake system is applied to one of the pistons to open one of the valveseats to atmosphere against the bias of the adjustable spring to depleteair from the spring chamber of the actuator in a ratio inverselyproportional to that which is applied to the service brake chambers ofthe brake actuators on the unfailed axle. Since some of the air in theactuator's spring brake chamber had been previously vented, the responsetime of the spring brake is rapid.

In accordance with another feature of the subject invention, the valveis balanced so that, in a brake failure mode, a greater pressure dropoccurs in the spring brake chamber than the service pressure applied tothe "unfailed" spring brake actuators. This is achieved by sizing thepressure responsive areas of the valve so that the area biasing one ofthe pistons from the third brake line is greater, by a predeterminedamount, than the pressure area biasing the same piston from the deliveryport.

In accordance with still another feature of the subject invention, thereis disclosed a modification to a conventional truck-trailer brake systemwhich meets safety criteria by supplying air at supply pressures to theemergency chambers of the tractor brake actuators and to the supply lineof the trailer while incorporating the inversion valve of the subjectinvention to sense and control the trailer emergency brakes as well asthe towing vehicle brakes when a brake failure occurs in the trailersystem or towing vehicle rear service brake system.

It is thus another object of the subject invention to provide in apneumatic brake system for a vehicle an inversion valve which ports airat supply pressure to the spring brake chamber in a normal operatingmode, but which automatically senses impending brake failure to reduceair pressure in the spring brake chamber to a minimum value to insurefast response time of the spring brake when an application is required.

Yet another object of the subject invention is to provide in a pneumaticbrake system for a vehicle, an inversion valve actuable upon a brakefailure to apply the brakes on the failed axle in a predeterminedbraking ratio to the braking force developed by the air operatedactuators on the other axles of the vehicle.

The invention may take physical form in certain parts and arrangement ofparts, a preferred embodiment of which will be described in detailherein and illustrated in the accompanying drawings which form a parthereof and wherein:

FIG. 1 is a schematic view of a typical pneumatic braking systememploying the inversion valve of the subject invention;

FIG. 2 is a sectional view of the inversion valve with the parts thereoforientated as they would appear without pressure in the vehicle airsystem;

FIG. 3 is an exploded sectional view of several components of theinversion valve;

FIG. 4 is a longitudinally sectioned view of the valve, similar to FIG.1, but with the component parts orientated as they would appear in alapped position of the valve; and

FIG. 5 is a tractor-trailer brake system including the inversion valveas one of its component parts.

Referring now to the drawings wherein the showings are for the purposeof illustrating a preferred embodiment of the invention only and not forthe purpose of limiting same, there is shown in FIG. 1 a pneumatic brakesystem 10 for use on a vehicle which incorporates an inversion valve 12of the subject invention having a first inlet port 30, a second inletport 31, a third inlet port 32, a delivery port 33 and a vent port 34(shown in FIG. 2).

Standard brake components shown in brake system 10 include a compressor13 charging reservoir "A" 14 and reservoir "B" 15 with air at supplypressure which in turn is applied through brake lines 17, 18 to theinlet side of a dual circuit brake valve 19. The designation "dualcircuit brake valve" is defined herein to include brake valves not onlyof the treadle type but also of the suspended pedal type and, inparticular, refers to such valves which utilize separate valvingmechanism to port air to the front and rear brakes of the vehicle. Inthe schematic illustrated, reservoir "A" air at supply pressure entersthe lower portion of dual circuit brake valve 19 and is ported ormodulated to service pressure at the outlet of valve 19 into brake line20. Brake line 20 in turn communicates air at service pressure to frontbrake actuators 21 (shown to be of the single diaphragm, air-applied,spring-released type) and also communicates air at service pressure tosecond inlet port 31 of inversion valve 12. Similarly, air at supplypressure from reservoir "B" in line 18 enters the upper portion of dualcircuit brake valve 19 where it is ported or modulated to servicepressure, leaving the outlet side of the valve through a brake line 23in turn in fluid communication with the rear brake actuators 24.

Rear brake actuators 24 are of the known dual diaphragm type and includea forward or service brake chamber 25 and a tandem rearward or emergencybrake chamber 26. Service brake chamber 25 normally brakes the rearwheels of the vehicle since it receives air at service pressure throughline 23 which displaces a diaphragm therein against a centrallysupported output shaft 27 which in turn rotates a conventional slackadjuster mechanism 28 to apply the rear brakes of the vehicle. Duringnormal highway operation of the vehicle, emergency brake chamber 26 issupplied air at supply pressure via brake line 36 in fluid communicationwith delivery port 33 of inversion valve 12 to maintain a spring 29precompressed by a diaphragm therein. When a failure occurs in thatportion of the brake system associated with reservoir "B" or when thevehicle is parked, the air at supply pressure in emergency brake chamber26 is vented to allow compression spring 29 to expand against outputshaft 27. The manner in which compression spring 29 is allowed to expandis dependent upon inversion valve 12 of the subject invention.

Completing the brake schematic is a brake line 37 "T'd" to reservoir "B"brake line 18 and connected to first inlet port 30 of inversion valve 12which, as explained hereafter, will function as a sensor means toregulate inversion valve 12. The third inlet port 32 of inversion valve12 is connected to brake line 38 in turn connected to a conventionalpark control valve 39 which always senses supply pressure by means of aconventional two way check valve 40 in fluid communication with eitherreservoir "A" or "B" depending upon which one is at higher pressure.

Referring now to FIGS. 2 and 3, inversion valve 12 is shown to include avalve body 42 having a stepped cylindrical bore 43 extending therein.First inlet port 30 is in fluid communication with bore 43 at one endthereof and bore 43 is closed at its opposite end by a vent cover 45threadably fastened as at 46 to valve body 42. Ribs 47 in vent cover 45engage the valve body's end portion opposite first inlet port 30 todefine a plurality of vent passages or vent ports 34 in fluidcommunication with bore 43. In between vent port 34 and first inlet port30 are second inlet port 31, third inlet port 32 and delivery port 33,all in fluid communication with bore 43. Disposed within bore 43adjacent vent port 34 is a first hollowed, cylindrically stepped tubularpiston 50. First piston 50 has a main body portion 51, an end portion 52stepped radially outwardly from main body portion 51 and generallyadjacent vent port 34. At the opposite end of main body portion 51,first piston 50 extends radially inwardly to define an annular shoulderseat surface 55 terminating in a longitudinally extending hollow stemportion 53 which in turn terminates in a flanged conical valve seat 54.The exterior of hollow stem portion 53 is stepped radially outwardly asat 56 and stem portion 53 extends into the interior of main body portion51 to define a boss 57 for retaining a spacer-washer 59 serving as aseat for a pair of compression springs 60 functioning as biasing meansto exert a bias to first piston 50 towards first inlet port 30. Firstpiston 50 is retained within bore 43 by sealing means in the form ofO-rings 63, 62 disposed within grooves located in main body portion 51and end portion 52 respectively of first piston 50. The area betweenfirst piston 50 and bore 43 enclosed by O-rings 62, 63 defines a firstpressure responsive area of the valve, hereindefined as "A-1.". The areacircumscribed by first valve seat 54, bore 43, first piston 50 andO-ring 63 is defined as the second pressure responsive area of valve 12,hereindefined as "A-2".

Disposed in bore 43 adjacent first inlet port 30 is a second piston orpiston means 65 defined as comprising a piston member 66, an end capmember 67 and a valve cage member 68.

Piston member 66 has a cylindrical base portion 70 at one end and aflanged end portion 71 at its opposite end. Capping the end of flangedend portion 71 is an annular seal 74 made of resilient material andhaving its outer periphery U-shaped as at 75 for sealing engagement withpiston member annular shoulder 73. Seal 74 is grasped about its outerperiphery by a metal cup-shaped annular retainer 76.

Base portion 70 of piston member 66 is adapted to be sealingly receivedwithin a centrally located, blind bore portion 78 of end cap member 67.End cap member 67 has a cylindrical main body portion 79, a flanged baseportion 80 adjacent first inlet 30 at one end of main body portion 79and a shouldered end 81 extending from the other end of cylindrical mainbody portion 79. Shouldered end 81 functions as a spring seat for oneend of a conical spring 82 which is seated at its opposite endunderneath cup-shaped member 76.

Flanged base portion 80 of end cap member 67 is lockingly engaged withina base portion 84 of valve cage member 68 by means of a snap ring 85.Valve cage member 68 is of tubular shape having a main body portion 86extending from base portion 84, indented radially inwardly in the areaof third inlet port 32 and having a plurality of openings or windows 87to permit air passage from third inlet port 32 to its interior.Extending from main body portion 86 is a forward portion 88 from whichextends a plurality of shouldered stops 89 extending in an annular arrayfrom forward portion 88 and adapted to contact shouldered seat surface55 of first piston 50. Extending radially inwardly from the interior offorward portion 88 is a frusto-conical second valve seat 90. Thediameter of second valve seat 90 is toleranced closely to the diameterof first valve seat 54 and, as shown in FIG. 2, is closely concentricwith stem portion 53. Formed in the interior of valve cage member 68 andextending from second valve seat 90 towards base portion 84 and aplurality of splines 92 having an internal diameter sized closely to theexternal diameter of cup-shaped retainer 76 for guiding piston member 66in its movement. The spaces between splines 92 define passages for airflow through the valve seat from third inlet port 32 to delivery port33. Sealing means for second piston 65 are provided in the form ofO-rings 95, 94 received within grooves formed in base and forwardportions 84, 88 respectively of valve cage member 68.

O-rings 94, 95 function as sealing means to define a third pressureresponsive area "A-3" of valve 12 specifically defined by that portionof bore 43 closed by O-rings 94, 95 and second valve seat 90. Also thatportion of the bore 43 closed by O-ring 95 adjacent first inlet port 30defines a fourth pressure responsive area "A-4." Bore 43 and thediameters of pistons 50, 65 are sized equally along their lands whichcontain O-rings 63, 94, 95. Pressure responsive areas "A-2" and "A-3"may be considered to be equivalent to one another and pressureresponsive area "A-1" is sized greater than pressure responsive areas"A-2," "A-3," preferably at a ratio of 1.5 to 1.

OPERATION

The operation of inversion valve 12 will first be explained withreference to the brake system shown in FIG. 1 and the vehicle in aparked position with a depressurized air system. In this mode,reservoirs "A" and "B" are assumed uncharged, with service brake lines20, 23 vented to atmosphere thus venting second inlet port 31 ofinversion valve 12. Park valve 39 is vented to atmosphere thus ventingbrake line 38 and third inlet port 32 to atmosphere. Similarly,reservoir "B" is not pressurized and little or no pressure exists inbrake line 37 and first inlet port 30 of inversion valve 12. With thepressures thus established, the component parts of inversion brake valve12 will assume the position shown in FIG. 2. With little or no pressureat first inlet port 30, the force exerted by compression spring 60 issufficient to bias first piston 50 downwardly in valve bore 43 sealingfirst valve seat 54 against seal 74 and contacting annular shouldersurfaces 55 with shoulder stops 89 forcing first and second pistons 50,65 to "column-up" until base portion 84 of cage member 68 contacts thebottom of valve bore 43. In this position, first valve seat 54 is sealedand second valve seat 90 is opened to permit air from emergency brakechamber 26 of the rear brake actuators 24 to vent to atmosphere viadelivery port 33 by traveling through pressure responsive area "A-3,"around second valve seat 90, through openings in splines 92, the windows87 in valve cage member 68 and from thence through third inlet port 32.

When the operator of the vehicle starts the engine, compressor 13automatically charges reservoirs "A" and "B" with air at supplypressure. Reservoir "B" air at supply pressure is then ported via lines18, 37 into first inlet port 30 and acting on area A-4 causes pistons50, 65 to move in a column, compressing spring 60, until end portion 52of first piston 50 contacts vent cover 45 which acts as a solid stop.First and second valve seats 54, 90 remain in their same relativeposition as previously described in a depressurized mode. When parkcontrol valve 39 is actuated, air at supply pressure from eitherreservoir "A" or "B" (whichever is higher) is supplied to third inletport 32 and travels through the valve in the path previously describedto delivery port 33, thence through brake line 36 into emergency brakechamber 26 to precompress actuator spring 29. In this position, thevalve is in its normal operating highway mode and so long as a brakefailure in the rear brake actuator system does not occur, air at supplypressure is ported to emergency brake chamber 26 of the rear brakeactuators thus insuring that the brake actuator springs 29 do not tendto partially apply the rear brakes of the vehicle. It should also benoted that service air brake applications to the vehicle with the valvein its normal highway operating mode do not affect the valve since firstpiston 50 is stopped from further travel by contact with vent cover 45.

Any brake failure in the rear axle brake system which results in a lossin pressure in reservoir "B" will trigger actuation of inversion valve12 in a manner now to be described. Reservoir "B" could lose pressure asa result of leakage or failure in reservoir "B" itself, or failure orrupture in any of the rear brake lines 18, 23, 37, or failure or leakagein dual circuit brake valve 19, or failure in rear brake actuators 24.If failure occurred in any of these components, pressure in first inletport 30 would drop. Since air at supply pressure exists at pressureresponsive area "A-2," the second piston would be forced downwardly inbore 43 while first piston 50 would remain biased against vent cover cap45. As the pistons separate from their columned-up position which theyassume in a normal operating highway mode, second valve seat 90 wouldmove closer into a contact engaging position with seal 74. If reservoir"B" pressure continues to drop, second valve seat 90 would contact seal74. Up to this point, conical spring 82 would maintain first valve seat54 in sealing engagement with seal 74. Further downward movement ofvalve cage member 68 will result in opening first valve seat 54 whilemaintaining second valve seat 90 sealed. During this movement, air atsupply pressure in third inlet port 32 is trapped within valve cagemember 68 and is not effective to bias second piston 65 in any directionwithin bore 43 while second valve seat 90 is sealed. Therefore, whenfirst valve seat 54 moves away from seal 74, air within emergency brakechamber 26 of spring brake actuator 24 will begin to vent through seat54, and vent ports 34 to atmosphere. Accordingly, the pressure withinpressure responsive area "A-2" of inversion valve 12 will drop untilsprings 60 overcome the force generated by air pressure acting on area"A-2" and move piston 50 downward. Equilibrium will occur when thepressure developed in area "A-2" exerts a force equal to the bias ofcompression spring 60. In this condition, the valve will be in a lappedposition such as shown in FIG. 4 with both valve seats 54, 90 seatedagainst seal 74. If there has been a complete failure in the pressure ofthe rear brake system, first piston 50 will be at the bottom of bore 43with the bias of springs 60 exerting a force against area "A-2" justsufficient to maintain compression spring 29 and brake actuators 24slightly compressed and valve 12 is now ready to cycle to an emergencymode for quick brake application. Under these conditions, it should benoted that a slight extension of the brake actuator output shaft, whichmay necessarily occur, is not viewed as a detriment since the conditionis not permanent.

If a brake application is now made by the vehicle operator, service airis delivered to second inlet port 31. The service air pressure actingagainst area "A-1" develops a force initially additive to that developedby emergency brake chamber air acting against area "A-2" and iseffective to move first piston 50 towards vent cover cap 45 openingdelivery port 33 to atmosphere via first valve seat 54. The pressure ofthe air in emergency brake chamber 26 is reduced to a lower value whichmultiplied by area "A-2" develops a force that is additive to thatdeveloped by service air pressure in area "A-1" to equalize the bias ofspring 60 whereat the valve returns to the lapped position. While areas"A-2" and "A-1" could be equal, it is desirable, for energyconsiderations, to have spring 29 of rear brake actuators 24 expand ortravel further against output shaft 27 to insure a brake applicationforce at rear brake actuators 24 at least equal to that developed by theunfailed front brake actuators 21. Thus pressure responsive area "A-1"is sized to be slightly greater than area "A-2" and preferably 1.5 timesas great. This permits the air pressure within emergency brake chamber26 to drop in pressure at a rate 1.5 times as great as that which isapplied by air at service pressure. While it is contemplated thatinversion valve 12 of the subject invention could be manufactured withvarious ratios of areas "A-1" and "A-2," a ratio higher than 1.5 to 1.0may not be desired. Generally speaking, spring brake torque must alwaysremain under the control of the vehicle operator. Assuming that theunfailed axles of the vehicle are equipped with antiskid devices orantilock controlled, a ratio higher than 1.5 to 1.0 may result in anoverly severe brake reaction from the spring brakes. In such instance,the spring brakes could lock the wheels they control while the wheelsunfailed under antilock control would not lock. It has been discoveredthat an inversion valve ratio of approximately 1.5 to 1.0 provides agood balanced brake reaction between unfailed and failed brake actuatorsalthough in antilock installations the inversion valve could adequatelyfunction at ratios less than 1.5 to 1.0.

After service brake application has been completed, second inlet 31 willbe vented to atmosphere by dual circuit brake valve 19 venting air frompressure responsive area "A-1." Springs 60 will force first valve seat54 against seal 74 compressing conical spring 82 to open second valveseat 90. This will establish fluid communication between third inletport 32 and delivery port 33. Pressure will build in area "A-2" andemergency brake actuator chamber 26 until equilibrium is reached withspring 60 whereat first piston 50 will move towards vent cover cap 45 toseat second valve seat 90 against seal 74 establishing a lapped positionof the valve and readying same to be triggered for the next brakeapplication.

The features and operating characteristics of inversion valve 12 of thesubject invention, as thus described, make inversion valve 12 suitablefor unique application to a tractor-trailer brake system. Suchapplication is shown in FIG. 5 which illustrates the uniquetractor-trailer brake system which offers safety advantages not possiblewith conventional air brake systems. As illustrated, a tractor brakesystem employing conventional antiskid or antilock system is shown onthe lefthand side of FIG. 5 and a conventional emergency relay typetrailer brake system is shown on the righthand side of the drawing,although it should be clear to those skilled in the art that the trailerwill function in the brake system illustrated if equipped with thestandard type of antiskid or antilock brake arrangement. With respect tothe tractor brake system illustrated, dotted lines refer to brake lineswith air at service pressure and solid lines refer to brake lines withair at supply pressure and like numbers with reference to FIG. 1 willdesignate like parts where applicable.

The conventional trailer system illustrated in FIG. 5 includes anemergency relay valve 340 which is connected to service and supply lines334, 335 respectively, a reservoir 341 and trailer brake actuators 343through suitable lines 344. Trailer brake actuators 343 are shown to beof the single diaphragm, air applied-spring released type although otherbrake actuators may be applied to the trailer system if slight changes,known to those skilled in the art, be made in the fluid communicationlines. Independent of the brake actuators employed, emergency relayvalve 340 functions in the usual manner to emit reservoir pressure tobrake actuators 343 when dual circuit valve 19 is depressed and ventsame when the dual circuit valve is released. Similarly, in the event ofa predetermined pressure drop in supply line 335, relay valve 340 isactuated to supply air at system pressure from reservoir 341 to trailerbrake actuators 343 to set the brakes. When supply pressure is restoredin line 335, relay valve 340 vents the air in trailer brake actuators343 to re-establish normal operating mode of the system.

With respect to the tractor brake system, the additional componentsillustrated therefor in FIG. 5 and not shown in FIG. 1 includeconventional skid control modulator valve 200 and appropriate brake lineplumbing associated therewith, known to those skilled in the art andthus not described in detail herein. Component valves shown in FIG. 5which render inversion valve 12 suitable for tractor-trailer applicationinclude a trailer control valve 201, a relay valve 202, a tractorprotection valve 203 and a governor valve 204, all these valves areknown to those skilled in the art and thus are not shown or described indetail herein.

Governor valve 204 is inserted in line 37 and functions as an on-offswitch controlling air to first inlet port 30 of inversion valve 12.Governor valve 204 is typically set at approximately 75-80 psi and solong as rear brake reservoirs "B" develop a pressure exceeding thisvalue, reservoir "B" will be in fluid communication with first inletport 30. When pressure in reservoir "B" drops below 75-80 psi, governorvalve 204 will act to prevent fluid communication between reservoir "B"and first inlet port 30 and will also drop the pressure at first inletport 30 to atmosphere through a vent mechanism provided in governorvalve 204 thereby rendering inversion valve 12 ready for complete springbrake application. The advantage of this will be explained later.

Trailer control valve 201 is similar in operation to park control valve39 and operates, upon application, to vent the trailer supply line ofair. The trailer emergency relay valve responds in a known manner toapply the trailer brakes when this line is vented. The inlet of trailercontrol valve 201 is in fluid communication with the outlet of two waycheck valve 40 and thus always senses air at supply pressure. The outletof trailer control valve 201 is in fluid communication with a brake line205 in turn in fluid communication with the reservoir port of relayvalve 202.

A brake line 206 in fluid communication at one end with delivery port 33of inversion valve 12 is in fluid communication with the control port ofrelay valve 202. Brake line 206 and control port of relay valve 202 maybe viewed as a line carrying a source of fluid at a signal pressure. Athird brake line 207 is in fluid communication with the outlet of relayvalve 202. When air at signal pressure exists in brake line 206, relayvalve 202 cycles to provide full fluid communication between brake lines207 and 205. When air pressure in line 206 drops below 75-80 psi (signalpressure), relay valve 202 is actuated to produce a similar drop inpressure between lines 205 and 207. When trailer control valve 201 isvented in an actuated position, line 205 is vented and relay valve 202receives no air at its reservoir port, therefore no delivery ispossible.

Brake line 207 is in fluid communication with the air supply line inletof tractor protection valve 203. Tractor protection valve 203 operatesto provide fluid communication with air at supply and service pressureson the tractor to that on the trailer so long as air at supply pressureenters its inlet side. If supply pressure air drops at the inlet side oftractor protection valve 203, tractor protection valve 203 cycles toprevent fluid communication of air at service pressure from the tractorside to the trailer side. A typical tractor protection valve will cycleto its "off" position when supply line pressure drops to approximately30-40 psi.

In operation and with reference to the previous description of theoperation of inversion valve 12, it should be clear that in the normalhighway operating mode, air at full supply pressure will enter thirdinlet port 32 and exit delivery port 33 to maintain springs 29 of thespring actuators 24 fully compressed. Tractor protection valve 203 willbe biased into its open position and the trailer supply line 335 will bepressurized in the normal manner.

A normal service brake application results in conventional response fromthe system. Tractor front service brakes are actuated by pressure inline 20. Rear service and trailer brakes are actuated by pressure inline 23. At this point it is important to note that the trailer supplyline is pressurized from lines 207 and 205 by way of valves 202, 201 and40. Air from either part of the tractor dual circuit brake system canfill this supply line. The trailer signal line receives pressure by wayof line 23 from valves 203 and 19 and as such is only able to drawpressure from reservoir "B." This feature has several advantages duringemergency stops when one or more components of the service brake airsystem have failed as discussed below.

One particular failure worth considering is a broken or disconnectedtrailer service line. This has serious consequences with conventionalsystems. The service line is unpressurized unless a brake application ismade. The open line goes unnoticed by a driver since no air escapes, butwhen a brake application is required, a massive leak occurs. Thisleakage rapidly drains a conventional tractor air brake systemdiminishing the tractor brake effectiveness, and since the trailerservice line is open, the trailer brakes remain inoperative. Even thenewer dual air brake systems do not correct this deficiency. With theproposed system, the vehicle driver retains control of the tractor andtrailer brakes and brake effectiveness is not greatly impaired. Thebrake application would initially result in massive leakage from theopen hose as before, except this leakage would only affect reservoir"B." Pressure in reservoir "B" would rapidly fall to 75-80 psi at whichtime governor 204 would function to exhaust port 30 of the inversionvalve. This causes the inversion valve to function as described above.The delivery pressure from the valve automatically drops to its presetemergency level or if the brake application is still being held, theinversion valve delivery will be further reduced by 1.5 times the amountof the service application. During this time, the tractor front brakeshave been fully active and the rear brakes active to the extent allowedby reservoir pressure at "B." The trailer brakes had been inoperative tothis point. The reduction of delivery pressure from the inversion valvechanges this situation. Reduced pressure in line 206 is sensed by relayvalve 202 and as a result pressure in line 207 drops quickly to the samelevel. Two events occur due to the pressure drop in line 207. First, thetractor protection valve 203 cycles in a known manner to close off thepassage between line 23 and the open trailer signal line. This stops theair leak from the tractor and retains 75-80 psi in reservoir "B." Thesecond reaction is by the trailer emergency relay valve. This valvefunctions in a known manner upon reduction of pressure in line 207 toautomatically apply the trailer emergency brakes. This series of eventsoccurs rapidly and automatically so that the braking performance of thevehicle is not greatly different from that experienced when the trailersignal hose is connected. Once the stop has been made and the braketreadle released, the inversion valve delivery pressure goes up to thepreset emergency level. This pressurizes line 206 thus cycling relayvalve 202 to permit supply pressure communication between lines 205 and207 thereby opening the tractor protection valve connection between thetractor and trailer to pressurize the trailer supply line for releasingthe trailer emergency brakes. The system response to the open trailersignal hose failure is thus far superior to that of conventionalsystems. This performance is made possibly by the full pressure deliverycharacteristic of the inversion valve which allows this valve to beinterposed in the trailer supply line. In general, a valve that deliversa regulated pressure lower than the supply pressure should not be usedin the towed vehicle supply line. Thus other known inversion controlvalves should not be used in this application.

The above description illustrates the functional interrelationships ofthe various valves in system shown by FIG. 5. Persons familiar withvehicle air brake systems will recognize that this system is capable ofstopping the vehicle under all manner of situations regardless offailures that may occur in one or more components and is, therefore,safer than conventional systems.

The invention has thus been described with reference to a preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the specification. For example,fasteners securing the vent cover to the valve body could be modified tobe adjustable so that the compression spring force could be adjustable.The valve parts could be inverted. The piston shapes changed and valveseal could be attached to the first piston. It is my intention toinclude all such modifications insofar as they come within the scope ofthe invention.

It is thus the essence of the invention to provide in a fluid actuated,vehicular brake system employing dual diaphragm brake actuators on atleast one axle of the vehicle, an inversion valve which is normallyeffective to maintain the springs in the brake actuators compressed atsupply air pressure and which is capable of rapidly applying the springbrakes of the brake actuators upon a system failure in a predeterminedratio to the braking force generated by other brake actuators employedon the vehicle.

Having thus defined the invention I claim:
 1. A valve for use with brakeactuators in a fluid-actuated braking system where sources of pressureat varying levels exist at valve ports comprising:a valve body having agenerally closed cylindrical bore therein, said body having a firstinlet port in fluid communication with one end of said bore, a vent portin fluid communication with the opposite end of said bore, second andthird inlet ports and a delivery port in fluid communication with saidbore; a first cylindrically stepped single-piece piston within said boregenerally adjacent said vent port, said first piston having a hollowstem in fluid communication at one end with said vent port and defininga first valve seat at its opposite end, said first piston includingsealing means circumferentially disposed thereabout to define with saidbore a first pressure responsive area in fluid communication with saidsecond inlet port and to define with said bore and said first valve seata second pressure responsive area said first pressure responsive areabeing at least as great as said second pressure responsive area but notgreater than 1.5 time said second pressure responsive area; biasingmeans contacting said first piston tending to move said first pistontoward said first inlet port; a second cylindrically stepped pistonwithin said bore generally adjacent said first inlet port, said secondpiston including a second valve seat facing and generally concentricwith said first valve seat, second sealing means circumferentiallydisposed about said second piston to define with said bore a thirdpressure responsive area in fluid communication with said first inletport and to define with said bore and said second valve seat a fourthpressure responsive area in fluid communication with said third inletport; sealing means associated with one of said pistons effective to (i)seal said first and second valve seats to prevent fluid communicationbetween said third inlet port, said vent port and said delivery portwhen said pressure at said outlet port acting over said second pressureresponsive area equals the pressure exerted by said biasing means andsaid pressure developed at said first inlet port is typically zero todefine a lapped position of said valve and (ii) sealing said first valveseat while opening said second valve seat to provide fluid communicationbetween said delivery port and said vent port at a rate equal to theratio of said first area and divided by said second area as pressure issupplied to said second inlet port to define a dump position of saidvalve; said valve moving from said dump position to said lapped positionwhen the pressure at said delivery port acting over the second area isequal to the pressure at said first port times the ratio of the secondarea divided by the first area.
 2. A valve for use with brake actuatorsin a fluid-actuated braking system where sources of pressure at varyinglevels exist at valve ports comprising:a valve body having a generallyclosed cylindrical bore therein, said body having a first inlet port influid communication with one end of said bore, a vent port in fluidcommunication with the opposite end of said bore, second and third inletports and a delivery port in fluid communication with said bore; a firstcylindrically stepped piston within said bore generally adjacent saidvent port, said first piston having a hollow stem in fluid communicationat one end with said vent port and defining a first valve seat at itsopposite end, said first piston including sealing meanscircumferentially disposed thereabout to define with said bore a firstpressure responsive area in fluid communication with said second inletport and to define with said bore and said first valve seat a secondpressure responsive area; biasing means contacting said first pistontending to move said first piston toward said first inlet port; a secondcylindrically stepped piston within said bore generally adjacent saidfirst inlet port, said second piston including a second valve seatfacing and generally concentric with said first valve seat, secondsealing means circumferentially disposed about said second piston todefine with said bore a third pressure responsive area in fluidcommunication with said first inlet port and to define with said boreand second valve seat a fourth pressure responsive area in fluidcommunication with said third inlet port; sealing means associated withone of said pistons effective to open and close said first and secondvalve seats; said second piston includes (a) a piston member having acylindrical base portion generally adjacent said first port and aflanged end portion opposite said base portion, (b) a generallycylindrical end cap having a central blind opening extending from oneside thereof for receiving in sealing engagement said base portion and(c) a tubular valve cage member in sealing engagement with said bore,said valve cage member sealingly receiving said end cap at one end andhaving a plurality of shoulders extending from its opposite end forcontacting said first piston, and said sealing means including aresilient member secured to said flanged portion of said piston member.3. The valve of claim 2 whereinsaid first piston has a main body portionapproximately equal in diameter to said end portions of said valve cagemember, an end portion stepped radially outwardly from said main bodyportion generally adjacent said vent port, an opposite end portionstepped radially inwardly from said main body portion and terminating insaid first valve seat, said valve cage member having a frusto-conicalsurface extending radially inwardly from the interior of said cagemember and at opposite end thereof to define said second valve seat, anda plurality of splines extending radially inwardly from the interior ofsaid cage member, said splines circumscribing and in guiding contactwith said seal.